why does my manual tell me my wing has a different CG for different motors?

ok my slow stick manual tell me that a 300 size motor the CG on the wing is at 110mm but a 400 size motor my CG should be located at 95mm. why is this? i plan on using a brushless motor with 35 ounces of static thrust should i move the CG even more forward on the wing? i just don't understand how the CG point can change because of a motor

Re: why does my manual tell me my wing has a different CG for different motors?

Quote:

Originally posted by jeff262 ok my slow stick manual tell me that a 300 size motor the CG on the wing is at 110mm but a 400 size motor my CG should be located at 95mm. why is this? i plan on using a brushless motor with 35 ounces of static thrust should i move the CG even more forward on the wing? i just don't understand how the CG point can change because of a motor

CG can vary due to wing loading. Difference at extreme ends of the envelope. As the s400 option will be considerably faster a more nose heavy CG will yield a less pitch sensitive model, which is desireable. At very low speeds and very light wing loading you need to increase pitch so a rearward CG is required.

That makes good sense. The 400 CG is at 33% and the 300 is at 36%. Just off the top of my head it does not seem that 1 ounce of added motor weight would make that much difference, but it may. The 33-36% values are a reasonable range; what I would do is start with the forward CG and move it back til the elevator felt "twitchy" and then move it forward a bit.

This is an important issue since many SS's are used for AP, which adds several ounces to the design weight. Again, I'd start at 33% and tweak.